Question

Solve the differential equation
$\dfrac{{dy}}{{dx}} + y\sec x = \tan x$, $0 \leqslant x \leqslant \dfrac{\pi }{2}$.

Answer 1

To solve this question firstly get to know about the differential equation and their types. After that, represent the given equation in the general form and then calculate the value of the integrating factor then put the value of the integrating factor in the formula used to solve the given equation.

Complete step by step solution:
The given question is about the differential equation so firstly let us try to understand the meaning of the differential equation.
The solution of а differential equаtiоn is, in generаl, аn equаtiоn exрressing the funсtiоnаl deрendenсe оf оne vаriаble uроn оne оr mоre оthers. Another way of sаying this is thаt the sоlutiоn оf а differential equation рroducer а function whаt саn be used tо рrediсt the behaviour of the оriginаl system, аt leаst within сertаin соnstrаints.
The most imроrtаnt саtegоries аre оrdinаry differentiаl equаtiоns аnd раrtiаl differentiаl equаtiоns. When the function invоlved in the equаtiоn deрends оn оnly а single vаriаble, its derivаtives аre known as оrdinаry derivаtives аnd that differentiаl equаtiоn is known аs аn оrdinаry differentiаl equаtiоn. Оn the оther hаnd, if the funсtiоn deрends оn severаl indeрendent vаriаbles, sо thаt its derivаtives аre known as раrtiаl derivаtives and the differentiаl equаtiоn is саlled аs а раrtiаl differentiаl equаtiоn.
The given differential equation is $\dfrac{{dy}}{{dx}} + y\sec x = \tan x$.
We can represent the given equation in the generalized form as,
$\dfrac{{dy}}{{dx}} + Py = Q$
Where $P = \sec x$ and $Q = \tan x$
To solve this type of problem we can use the formula
$y \times IF = \int {\left( {Q \times IF} \right)dx + C}$ $........(1)$
Where $IF$ is an integrating factor.
$C$ represents the integration constant.
Now let us try to evaluate the value of the integrating factor using the formula:
$IF = {e^{\int {Pdx} }}$
Substitute the value of $P$as $\sec x$, we get
$\Rightarrow IF = {e^{\int {\sec xdx} }}$
As we know that the integration of $\sec x$ is $\log \left| {\sec x + \tan x} \right|$
$\Rightarrow IF = {e^{\log \left| {\sec x + \tan x} \right|}}$
$\Rightarrow IF = \left| {\sec x + \tan x} \right|$
Now put the value of the integrating factor in the equation $(1)$, we get
$y \times IF = \int {\left( {Q \times IF} \right)dx + C}$
$\Rightarrow y(\sec x + \tan x) = \int {(\tan x(\sec x + \tan x))dx + C}$
$\Rightarrow y(\sec x + \tan x) = \int {\tan x\sec xdx + \int {{{\tan }^2}xdx + C} }$
As we know that the integration of $\tan x\sec x$ is $\sec x$and we can represent ${\tan ^2}x$ as ${\sec ^2}x - 1$with the help of basic identity, ${\tan ^2}x + {\sec ^2}x = 1$
$\Rightarrow y(\sec x + \tan x) = \sec x + \int {({{\sec }^2}x - 1)dx + C}$
$\Rightarrow y(\sec x + \tan x) = \sec x + \tan x - x + C$
So we can conclude that the final answer is $\sec x + \tan x - x + C$.

Note: In the above problem, we have used some trigonometric identities to simplify the terms. One should be more careful in choosing the identities since we have more than one formula for some trigonometric functions, so we must choose the appropriate one for our calculation. Choosing the appropriate formula may lead to an endless process of calculation where we cannot find the solution.